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In HDFS, reads normally go through the DataNode. Thus, when the client asks the DataNode to read a file, the DataNode reads that file off of the disk and sends the data to the client over a TCP socket. So-called "short-circuit" reads bypass the DataNode, allowing the client to read the file directly. Obviously, this is only possible in cases where the client is co-located with the data. Short-circuit reads provide a substantial performance boost to many applications.
To configure short-circuit local reads, you will need to enable libhadoop.so. See Native Libraries for details on enabling this library.
Short-circuit reads make use of a UNIX domain socket. This is a special path in the filesystem that allows the client and the DataNodes to communicate. You will need to set a path to this socket. The DataNode needs to be able to create this path. On the other hand, it should not be possible for any user except the hdfs user or root to create this path. For this reason, paths under /var/run or /var/lib are often used.
Short-circuit local reads need to be configured on both the DataNode and the client.
Here is an example configuration.
<configuration> <property> <name>dfs.client.read.shortcircuit</name> <value>true</value> </property> <property> <name>dfs.domain.socket.path</name> <value>/var/lib/hadoop-hdfs/dn_socket</value> </property> </configuration>
This configuration parameter turns on short-circuit local reads.
If this configuration parameter is set, short-circuit local reads will skip checksums. This is normally not recommended, but it may be useful for special setups. You might consider using this if you are doing your own checksumming outside of HDFS.
The DFSClient maintains a cache of recently opened file descriptors. This parameter controls the size of that cache. Setting this higher will use more file descriptors, but potentially provide better performance on workloads involving lots of seeks.
This controls the minimum amount of time file descriptors need to sit in the FileInputStreamCache before they can be closed for being inactive for too long.
This control whether we will try to pass normal data traffic over UNIX domain sockets.